The embodiments herein relate generally to mineral insulated electrical cables, and more particularly, to a method for extending both the operating life and range of medium voltage mineral insulated electrical cables in high temperature environments.
Mineral insulated cables over 2000 feet long, when operated in the temperature range of from about 200° C. to about 700° C., aside from manufacturing or installation defects, tend to fail due to one or two contributing causes. Magnesium oxide is the most prevalent cable insulant of mineral insulated cables in used today. It is well documented that magnesium oxide insulated cables' impedance decreases with increasing temperature. By Ohm's Law, for a given operating voltage, the leakage current from the cable conductor through the insulant will increase in proportion to the impedance decrease; this increased leakage current over time can result in cable failure. To achieve reliable operation in high temperature environments, applications typically limit the operating voltage based upon a safety factor of about 2 to 3 times the dielectric rating of the mineral insulated cable. This voltage derating either limits the application length or requires a thicker insulated cable at a higher cost; either of which affects the economic viability of applications, especially in the oil industry down-hole heater applications.
Another typical mineral insulated cable failure is instantaneous voltage breakdown failure when the cable is exposed to an overvoltage condition exceeding the dielectric rating of the cable insulant material. The overvoltage can be due to external causes, such as switching operations or lighting impulse propagation from the heater power source to the cable. An overvoltage condition can also be created by the reflecting voltage waves caused by the PWM voltage pulses of the Variable Speed Drive, common in motor applications with long feeder cables. Even with the typical drive output filter, the reflecting voltage waves can reach amplitudes multiple times that of the normal operating voltage, especially with cable lengths over 2000 feet, typical in the oil industry applications of mineral insulated cable as down-hole heaters.
Therefore, what is needed is a method to improve the reliability and range of use of mineral insulated electrical cables in high temperature environments.